Fix commit.File() gorutine leak

[alcortesm]

Commit.File() was leaking a goroutine because it was looping over
an iterator without closing its channel.

Now commit.File() calls the new Tree.File() method that searches
the file in the repository by trasversing the dir tree instead
of using the tree.Files() iterator.

This not only prevent the goroutine leak, but also speeds up
file searching.

[codecov-io]

Current coverage is 69.51%

Merging #14 into master will increase coverage by +1.14% as of 827470d

@@            master     #14   diff @@
======================================
  Files           17      17       
  Stmts         1208    1253    +45
  Branches       192     200     +8
  Methods          0       0       
======================================
+ Hit            826     871    +45
  Partial         93      93       
  Missed         289     289       

Review entire Coverage Diff as of 827470d

Powered by Codecov. Updated on successful CI builds.

[toqueteos]

This is a public method, it needs a comment 🎉

[toqueteos]

LGTM